What if forests became grasslands?

In a forest, tall trees grow close together. In a grassland, grass waves in the wind and there are few trees.

What if a forest slowly turned into a grassland? What would change?

Forests and grasslands look completely different, but in many places they meet and even swap places over time. A third of North America was once covered by grassland, called prairie, and in many places forests and prairies have pushed back and forth as fires, grazing animals and weather changed.

This lab is a thought experiment. Imagine a forest becoming a grassland. Choose a path to explore the animals, the soil and fire, and the carbon stored in the trees.

Grassland once covered about a third of North America. The boundary between prairie and forest is not fixed: the National Park Service describes the species makeup as set by climate, fire frequency and the degree and frequency of grazing.

This thought experiment imagines a forest converting to grassland and asks three questions: which animals would the new habitat support, how would fire and soil change, and what would happen to the carbon stored in wood and leaves?

Forest animals climb trees and hide in the shade.

Grassland animals, like bison and prairie dogs, eat grass out in the open.

A forest stores much of its life above ground, in trunks, branches and leaves. A grassland is the opposite: more than half of a grass plant's mass is under the ground, in its roots. Some prairie grasses have roots 12 feet deep or more.

Grasslands are far from empty. Bison, elk, deer, pronghorn and prairie dogs graze there, and hawks, coyotes and mountain lions hunt. Tallgrass prairies get 30 inches of rain or more a year; shortgrass prairies get only about 12.

Grasses put more than half of their mass below ground. Their roots build and hold soil and store food for winter and drought, and silica in their cells makes the leaves coarse and less palatable, an adaptation to grazing. Some tallgrass prairie species root 12 feet deep or more.

In the Great Plains, rainfall ranges from 30 or more inches a year in tallgrass prairie to about 12 in shortgrass prairie, with mixed-grass between. The prairie supports grazers such as bison, elk, deer, pronghorn and prairie dogs, and predators from birds of prey to black-footed ferrets.

Imagine the trees are gone and grass grows instead.

Animals that need trees move away. Animals that eat grass move in. The ground is sunny now.

If a forest became a grassland, sunlight would reach the ground, and grasses and wildflowers would spread. Animals that nest or feed in trees would have to find forests elsewhere, while grazers such as bison and prairie dogs could move in. That part is an idea based on what each habitat supports.

Fire would matter much more. On the prairie, fires burn old dead grass, but most of each grass plant survives underground and grows back quickly.

A conversion would open the canopy, shifting the community toward grasses, wildflowers and grazers; forest-dependent species would lose habitat. This is reasoning from the habitats' known communities, not an observed conversion.

Fire regimes would change too. Prairie fires remove the litter of past years, release nutrients into ash and kill competing trees and shrubs, while grasses survive below ground and benefit from the extra light, water and space.

Fire, grazing animals and dry weather help keep grasslands grassy.

Without fire, little trees can grow up, and a grassland can slowly become a forest.

Grasslands stay grassy because of fire, grazing and dry spells. Fire kills young trees and shrubs but not the grass roots. Without fire, tree seedlings sprout on the prairie, grow up, and forest takes over. The National Park Service says eastern redcedar can turn a grassland into a forest within 40 years without fire.

So the reverse, a forest becoming grassland, would need drier conditions, more fire or more grazing. That's why park managers light careful, planned fires called prescribed burns.

The NPS states that grasslands in the Midwest thrive with fire, grazing and drought, and that without regular fire park grasslands become shrublands or woodlands; eastern redcedar alone can convert grassland to forest within 40 years. In the Black Hills, fire suppression has raised forest density, reduced plant diversity and fed more intense fires.

By symmetry, forest-to-grassland change requires the opposite pressures: drier climate, frequent fire or heavy grazing. Prescribed fire, lit only when a written 'prescription' of weather and conditions is met, is how managers hold the balance today.

Trees keep lots of carbon in their trunks. Grass keeps carbon in its roots, under the ground.

People take care of both forests and grasslands, because both are important.

Plants take carbon dioxide from the air and store the carbon. Forests store most of theirs in wood and leaves; grasslands store most of theirs underground, in roots and soil. When a forest burns, much of its carbon goes back into the air. When a grassland burns, the carbon underground mostly stays put.

In a stable climate, trees store more carbon than grasslands. But a University of California, Davis study found that in drought- and fire-prone California, grasslands are a more reliable carbon store. Only a tiny sliver of the old tallgrass prairie remains, by some estimates less than 1 or 2 percent,, so people protect and restore both habitats.

A UC Davis study (Dass et al., Environmental Research Letters) modeled California ecosystems and found grasslands and rangelands to be more resilient carbon sinks than forests this century, because grassland carbon sits mostly below ground and survives fire, while forest carbon in wood and leaves is released when trees burn. Its co-author notes that in a stable climate trees store more carbon than grasslands.

By that reasoning, turning forest into grassland might release much of the carbon stored above ground, while possibly creating a more fire-proof store below. Meanwhile, the original tallgrass prairie has shrunk to perhaps 1–2 percent (Pipestone) or as little as 4 percent (Heartland Network), mostly converted to farmland.